1. Field of the Invention
The present invention relates to a laser drawing apparatus for drawing a pattern on a surface of a workpiece by scanning the workpiece surface with laser beams and then modulating the laser beams, on the basis of raster-graphic data, in accordance with a series of clock pulses.
2. Description of the Related Art
Such a laser drawing apparatus is used for drawing a fine pattern on a surface of a suitable workpiece. As a representative use of the laser drawing apparatus, it is known that a fine circuit pattern is drawn on the workpiece when producing a printed circuit board using photolithography. In this case, the workpiece may be either a photosensitive film for producing a photomask or a photoresist layer formed on a suitable substrate.
Recently, a circuit pattern drawing system has been developed for efficiently carrying out a drawing of a circuit pattern by integrating the use of a laser drawing apparatus, a CAD (Computer Aided Design) station for designing a circuit pattern, a CAM (Computer Aided Manufacturing) station for editing the designed circuit, an EWS (engineering work station) for commanding and controlling the drawing operation of the individual laser drawing apparatus and, finally, a LAN (local area network) through which the CAD station, the CAM station, the EWS and the laser drawing apparatuses are all connected to each other.
A circuit pattern is designed and treated as vector-graphic data by the CAD station. The vector-graphic data may be fed from the CAD station to the CAM station, through the LAN, for editing of the designed circuit pattern, if necessary. In either case, the vector-graphic data is fed from the CAD station or CAM station to the EWS, through the LAN, and is stored in a storage medium, such as a hard disk, provided in the EWS.
The EWS suitably feeds the vector-graphic data to one of the laser drawing apparatuses, and the vector-graphic data are converted into raster-graphic data by a vector-to-raster graphic converter provided in the laser drawing apparatus. The raster-graphic data are developed and stored in a pattern memory or bit-map memory and, in a drawing operation, the raster-graphic data are successively read from the bit-map memory.
The laser drawing apparatus comprises a drawing table on which the workpiece is positioned. Also, the laser drawing apparatus comprises a laser beam generator for emitting laser beams, and a laser beam deflector, such as a polygon mirror, for deflecting the laser beams in a main-scanning direction, so that the workpiece placed on the drawing table is scanned with the deflected laser beams. In the drawing operation, the drawing table is moved in a sub-scanning direction, which is perpendicular to the main-scanning direction. The movement of the drawing table is carried out by a drive motor, such as a stepping motor, a servo motor and so on, which is driven in accordance with a series of drive pulses. The deflected laser beams are modulated on the basis of the read raster-graphic data, and the modulation of the laser beams is performed in accordance with a series of clock pulses. Thus, a circuit pattern, based on the raster-graphic data developed in the bit-map memory, is drawn on the workpiece.
The circuit pattern drawn on the workpiece is formed by pixel-dots which are produced along main-scanning-lines, defined on the workpiece by the deflection of the laser beams in the main-scanning direction, and along sub-scanning-lines defined on the workpiece by the movement of the drawing table in the sub-scanning direction. Of course, the production of the pixel-dots is dependent upon the modulation of the laser beams based upon the raster-graphic data, and a pitch of the produced pixel-dots depends upon a frequency of the series of clock pulses.
Ideally, the pixel-dots should be uniformly arranged at a constant pitch along each of the main-scanning-lines and along each of the sub-scanning-lines before the drawn circuit pattern can be obtained with a high quality. Nevertheless, in laser drawing apparatuses, it is impossible to obtain a uniform arrangement of the pixel-dots along the main-scanning-line and the sub-scanning-lines due to assembly error caused during manufacture of the laser drawing apparatus.
Conventionally, after a laser drawing apparatus is assembled, a drawing operation is actually executed in which a test pattern is drawn, and an irregularity of the arrangement of the pixel-dots is measured with respect to the test pattern. Then, a pixel-pitch correction is introduced into the vector-graphic data in the CAM station, on the basis of the measured irregularity of the arrangement of the pixel-dots, so that the pixel-dots can be arranged as uniformly as possible along the main-scanning-lines and along the sub-scanning-lines. Nevertheless, the pixel-pitch correction is merely performed with a unit of one-pixel-dot size with respect to a pattern to be drawn. Namely, in the conventional laser drawing apparatus, it is impossible to perform a precise pixel-pitch correction with a unit of less than one-pixel-dot size.
Alternatively, a precise pixel-pitch correction with a unit of less than one-pixel-dot size can be performed by adjusting both an output-timing of the clock pulses, for the modulation of the laser beams, and an output-timing of the drive pulses, for the drive motor of the drawing table, in accordance with the measured irregularity of the arrangement of the pixel-dots.
In particular, the clock pulses for the modulation of the laser beams are derived from the divisions of a linear scale provided along the main-scanning direction, and the drive pulses for the drive motor of the drawing table are derived from the divisions of a linear scale provided along the sub-scanning direction. If both the divisions of the linear scales are modified in accordance with the measured irregularity of the arrangement of the pixel-dots, a precise pixel-pitch correction with a unit of less than the one-pixel-dot size is made possible. Nevertheless, this approach is unadvisable because sets of the two linear scales, each having the modified divisions, must be prepared with respect to individual laser drawing apparatuses, and this becomes very costly.
Further, in the laser drawing apparatus as mentioned above, a drive mechanism for driving the drawing table is especially susceptible to assembly error, resulting in a disordering of the arrangement of the pixel-dots along the sub-scanning-lines. For example, although the sub-scanning-lines, along each of which the pixel-dots are arranged, should be precisely extended perpendicular to the main-scanning-lines, the sub-scanning-lines may be inclined with respect to the main-scanning-lines due to an assembly error during manufacture of the drive mechanism for driving the drawing table. Further, although the pixel-dots should also be precisely aligned with each other along each of the sub-scanning-lines, a misalignment of pixel-dots along the sub-scanning-lines may also be produced due to an assembly error during manufacture of the drive mechanism for driving the drawing table.
Similar to the conventional case as mentioned above, an irregularity of the arrangement of the pixel-dots along the sub-scanning-lines is measured by actually executing a drawing operation of a test pattern using an assembled laser drawing apparatus. If the measured results represent an inclination of the sub-scanning-lines with respect to the main-scanning-lines, an inclination correction, based upon the measured results, is introduced into the vector-graphic data in the CAM station so that the inclination of the sub-scanning-lines can be improved as much as possible.
If the measured results represent a misalignment of the pixel-dots along the sub-scanning-lines, a misalignment correction, based upon the measured results, is introduced into the vector-graphic data in the CAM station so that the misalignment of the pixel-dots along the sub-scanning-lines can be improved as much as possible. Nevertheless, the inclination correction and the misalignment corrections are merely performed with a unit of one-pixel-dot size with respect to a pattern to be drawn. Namely, in the conventional laser drawing apparatus, it is impossible to perform a precise inclination correction and a precise misalignment correction with a unit of less than one-pixel-dot size.